Abstract

Late accretion, early mantle differentiation, and core-mantle interaction are processes that could have created subtle 182W isotopic heterogeneities within Earth’s mantle. Tungsten isotopic data for Kostomuksha komatiites dated at 2.8 billion years ago show a well-resolved 182W excess relative to modern terrestrial samples, whereas data for Komati komatiites dated at 3.5 billion years ago show no such excess. Combined 182W, 186,187Os, and 142,143Nd isotopic data indicate that the mantle source of the Kostomuksha komatiites included material from a primordial reservoir that represents either a deep mantle region that underwent metal-silicate equilibration or a product of large-scale magmatic differentiation of the mantle. The preservation, until at least 2.8 billion years ago, of this reservoir—which likely formed within the first 30 million years of solar system history—indicates that the mantle may have never been well mixed.